Boost converters require a start-up procedure that brings an input voltage up to a certain level before the operation of the boost converter can start. Conventionally, when a boost converter is started, the output voltage is lower than the input voltage by a diode voltage drop, due to the forward body diode of the high side switch. However, some applications such as displays require the output voltage to rise to the desired level starting from zero. In this case the output must be properly regulated during the start-up phase in order to avoid inrush currents that might damage the electrical circuits of the boost converter and of the devices coupled to the converter. This issue has generally been addressed by configuring the boost converter to implement a dual-mode operation such that at start-up the boost converter operates according to a “soft start-up” method and after start-up the boost converter operates according to normal “boost switching” operation.
A common approach for implementing soft start-up in boost converters consists in implementing a start-up procedure that comprises a start-up pre-charge phase followed by a start-up-boost phase. During the start-up pre-charge phase, also referred to as pre-charge phase, the output voltage increases gradually from zero to a voltage equal or close to the input voltage. During the start-up boost phase the boost circuit is turned on and the output voltage is pumped up to the target voltage via the switching operation of the converter, after which the converter enters the normal boost switching operation.
N-N type boost converters are a class of boost converters having a low side and a high side switch implemented as N-type switches, such as N-type MOSFET switches. For some applications, such as display, the N-N topology is becoming increasingly popular over the P-N topology, where the low side switch is implemented by a N-type MOSFET switch and the high side switch by a P-type MOSFET switch. This is mostly due to the area savings achievable by using N-type switches without reduction in performance.
However, implementing a pre-charge phase to achieve soft start-up in N-N boost converters is challenging. In order to gradually increase the output voltage of N-N boost converters from zero to the desired level while preventing the inductor current from increasing excessively, a specifically design pre-charge system and procedure are generally required.
Some N-N boost converters implement the pre-charge phase by using a current limiting switch and a charge pump to gradually increase the output voltage to the same level as the input voltage before entering the start-up boost phase. However, this solution entails higher design costs and complexity as well as a larger implementation area, which is incompatible with small devices or wearable applications where area savings are essential for an optimized product design and customer experience.
Other prior art start-up N-N boost converters do not make use of a charge pump, however they do not manage to raise the output voltage to the input level before entering the start-up boost phase and therefore when the boost phase starts they are affected by an initial inrush current limiting the performance of the converter.